One approach for head-mounted virtual reality displays has been to use a Ferrand Pancake Window™ as described in U.S. Pat. No. 3,443,858 and follow-on devices which simply make the design more light efficient. This design is shown in FIG. 1. Underlying this design is the recognition that a curved mirror can be used to achieve short focal lengths with large diameters thus offering wide fields of view. Additionally, the pancake window can be made to be semi-transparent by replacing the display with a 45 degree 50% reflective mirror and additional optics in front of the display which is folded out of the optical path to form a real image of the display where the pancake is focused; the refractive path through the pancake and the 45 degree mirror offer a view into the real world and because the power from the pancake comes from the reflective surface, it can be buried in a plano-plano doublet or as an interior partially reflective surface. The optical path of the pancake window device uses circular polarization based mirror bounces to generate a virtual image of the display seen from the eye side of the optics. The eye sees light which was, during the optical path, traveling away from the eye.